This paper presents a sequence of signal processing and hierarchical clustering techniques utilized to process signals with low signal-to-noise ratio (SNR) measured by multiple AE sensors. Noise and other extraneous events present major challenges for the detection and monitoring of AE signals generated by the inception of microcracks and their growth. Characteristics of AE waveforms released during controlled mode-I fracture are explored, and these characteristics are used for clustering AE and locating the fracture. With hierarchical clustering and signal denoising techniques, it is possible to locate the position of the crack plane with high accuracy using AE signals of poor quality, wherein the spatial distribution of clusters is indicative of crack propagation.

Acoustic emission (AE) measurement was used to assess the behavior of concrete beams subjected to flexural loading. Plain concrete specimens and those with vinyl fiber-reinforced mortar layer as composite were prepared. Discussions are based on utilizing various AE parameters for investigating the fracture behavior of composite specimens from those of plain concrete specimens, as well as assessing locations of cracking and occurrence of debonding at interface between concrete and fiber-reinforced mortar layer. Part of the assessment was justified by visual inspection carried out during the loading tests. Results in general indicated that in terms of fracture development, the composite specimens behaved similarly to the plain concrete specimen, with higher flexural strength and possibly higher shear resistance as suggested by AE parameters. Besides, there was no significant interfacial debonding observed throughout the flexural tests, inferring that the bond stayed intact during the test, such that the specimens were able to sustain stress sufficiently as a composite element. The AE parameters were found useful in distinguishing between the flexure and shear fracture modes and thus can be utilized as indicators to predict fracture behavior of concrete structures in health monitoring process.

As the fundamental research of rock fracture, we have simulated the uniaxial compression test of rock using the distinct element method (DEM) and discussed the influence of the slip at crack surface to a relative number of AE events. Simulation results agree well with the AE activities observed in an actual experiment and provide new findings to resolve the disagreement; the conventional theories and microscopic observations suggest that tensile cracks cause AE events, whereas an abundance of shear AE events is observed in experiments. Our simulation results indicate that the energy released from a tensile microcrack is very small and is most likely buried in noise compared with that from a shear crack, which should be observed predominantly, due to much smaller tensile strength compared to compressive strength. Further, AE is mainly generated from new tensile microcracks when the stress level is low, while the main sources of AE shift to the slip at the existing crack surface as the macroscopic failure approaches. That is, the burst of AE events during the formation of macroscopic fracture is from the slip occurrence at the existing crack surface. The results indicate that DEM is an effective numerical analysis technique for studying the dynamics of microcracking in brittle materials like rock.

We propose a new sensor testing technique, which may be a good candidate for the absolute sensor calibration, for the routine laboratory and in-field sensor response checking and for selecting the sensors with similar responses for AE source location problems. We suggest utilizing the energy of the high power electromagnetic field in a coaxial transfer line with a specific geometry to excite a mechanical wave at the surface of a conducting media. Theoretical modeling approach is discussed and the experimental validation of the proposed method is presented. Advantages of this method are as follows: i) the extremely high stability of the elastic wave at the epicentral point where the AE sensor is attached; ii) the field of mechanical displacements produced by the magnetic field pressure is computable as a function of time with an aid of finite element method; iii) dimensions are small, installation is easy and convenient both for the academic laboratory experiments and for the everyday NDT practice; iv) low cost and simple maintenance.

Clogging of pipes caused by bivalves such as mussels is a serious problem preventing safe operation of plants. Effective early detection of mussel clogging was studied using an optical fiber AE system. This system was developed to detect minimum flow velocities when AE signals are generated from mussels. First, a sheet-type optical fiber sensor was developed for the detection of cylinder-wave AE signals from mussels. The sensor was used by winding it around a pipe. The frequency response of 13 kHz to 27 kHz from the developed sensor depended on its width. AE signals from living mussels attached on the inside surface of PMMA pipe were monitored next. The flow velocity when the first AE signal was detected increased depending on the shellfish size. AE signals were produced by mussels that were more than 11 mm long. AE signals from mussel colony were than monitored. The flow velocity, when the first AE signal was detected, was also dependent on shell size. However, the flow velocity was lower than that of the single mussel test and mussels that were less than 5 mm produced AE signals. Additionally, the flow velocity decreased linearly with the shell length of colony members. We identified the minimum mussel size for AE detection for a given flow velocity.

CUI (corrosion under insulation) of the piping at industrial plants gathers more attention than ever. Currently, plant owners need to shut down their operation, scaffold, disassemble insulation, carry out non-destructive test and reassemble insulation of extensive piping installation. On-stream inspection (OSI), or on-line monitoring is a key to improve economics. To evaluate CUI without plant shutdown, we have carried out a preliminary research on detecting AE produced by corrosion. Fiber optic AE sensor is explosion proof, and is suitable for applications in petrochemical plants. Evaluation testing was successful, and one sensor can detect corrosion 3.9 m away. We report experimental results and subsequent field test, using fiber optic AE sensor.

This research aims to study the effect of shot peening on the delayed fracture using the Almen strips and AE technique. The Almen strip is a thin spring-steel coupon for measuring the peening intensity from its arc height. We used the conventional delayed-fracture test of three-point bent strips in two types of charging solutions with and without poison (thiourea) and step-wise strain increase (SSI) method. AE technique was successfully utilized to determine the threshold strain to induce the subsurface micro-cracks in the strips. Proposed test method was found to be valid for high strength steels (>1 GPa) with trapped hydrogen, and provides us with important information on hydrogen.

Failure of high-strength steel cables of prestressed-concrete structures (PCS) is becoming a serious problem, as the deterioration of structures progresses due to corrosion induced in severe environment as chloride attack. This attack is dangerous due to the fact that these cables are tensioned at 80% UTS in concrete, so a stress-corrosion cracking (SCC) mechanism can be developed in this condition causing steel brittle failure without any external warning. The applicability of acoustic emission (AE) technique for evaluation and detection of SCC and localized corrosion of steel cables in simulated concrete-pore solution (0.01M of NaOH at high alkalinity) contaminated by chloride ions (0.1M) is studied. Tests performed in laboratory show that the cracking process can be practically monitored by AE, as well as wires failure under constant load deformation. A novel analysis of AE parameters using the principal component analysis (PCA) is used to discriminate localized corrosion from SCC. Kmean is used first as unsupervised method, and to validate the clustering k-nearest neighbor is used as supervised method. Among this study, the AE monitoring of cable corrosion was proven to be useful under severe load condition.

It is important to interpret and evaluate the fracture mechanism of deteriorated concrete structures with rebar corrosion. Monitoring by means of acoustic emission (AE) technique based on the clarified fracture process of the deteriorated concrete is also useful for maintenance scheme. This paper presents an experimental investigation on the flexural failure behavior of RC beams having different weight loss of 0, 5, 10 and 30% due to corrosion. AE was also monitored during the loading tests of the deteriorated concrete beams. As corrosion levels represented by weight loss increased, load carrying capacity of the beams decreased dramatically. In the case of the severest damage having weight loss of 30%, the rebar was eventually broken at the final stage. The mechanical behavior was attributed to the decrease of cross section of rebar and deterioration of bond properties (debonding) between concrete and rebar with corrosion. The deterioration of bond properties also provided the decrease of number of cracks within the beams. Regarding observation of AE activity, the number of AE events in the beams increased with increasing the corrosion levels, especially as it became more intense at the initial loading level before yielding of rebar. The bond deterioration in the beams with corrosion might occur around the rebar at an early loading stage. The AE sensors attached directly onto the rebar detected more AE signals with lower frequency than that of concrete. It was concluded that such detected AE signals with lower frequency appear to be generated by debonding behavior due to corrosion.

The paper describes the examples of AE method in industry for detecting and locating leaks in constructions of double bottom storage tanks. The tests were made on new and modernized tanks in various test conditions and utilized different ways of defect location. It presents also results of laboratory tests to evaluate the possibility of detecting corrosion processes in this inaccessible space of double bottom.

Recently, the corrosion evaluation technology of tank floors by acoustic emission (AE) method has been put to practice in Japan. However, the corrosion evaluation with the AE method requires the judgments of the influence of various noises, and this factor decreases the accuracy of this evaluation technology. In this research, we studied the discrimination and removal methods of noise due to condensate dropping on the surface of the stored medium. We identified noise sources with three-dimensional source location (3D source location), verified a feature of the waveform of the drop noise, and confirmed the effects of some removal methods for the drop noise.

Acoustic emission (AE) source location is an essential part of any quantitative AE test as it provides information about damage mechanisms and allows spatial separation so that signals from unwanted sources can be eliminated. In this paper, an AE source location technique described as the best-matched point search method is presented. The application of the bestmatched point search method is demonstrated in two source location experiments: one on a large anisotropic carbon-fibre composite (CFC) plate and one on a thick oolitic limestone disc. In the large composite plate test, source location is achieved using the S0 mode, which displays a complicated group velocity pattern. In the oolitic limestone experiment, three-dimensional source location is demonstrated. The best-matched point search method successfully determines the location of AE sources in both tests. Errors in source location are attributed to the extraction of delta-t times from the AE signals.

For decades now, acoustic emission (AE) testing of pressure vessels has been used in France, Europe and the rest of the world. There are several regulatory rules, codes and standards worldwide, which define the application rules of this method. Since 2004, France has officially adopted a Best Practices Guideline [1] used as a reference for customers and service providers to apply this technology to various pressure vessels. According to the Guideline, like several other European (European standards) or American (ASME) codes, AE testing can be applied based on two techniques (zonal location method and planar source location by triangulation method). However, no comparative study of their performance, thus enabling their assessment, has been carried out. By means of simple simulation calculations, this study highlights the significant differences in performance between these two techniques. The effects of other fundamental parameters, for example, acquisition threshold, are also quantified with respect to AE usage. This study may also be used as a basis for defining a new AE testing standard specifically and quantitatively defining the expected performance of a given configuration. Today, the CETIM may apply this new testing methodology based on significant feedback enabling a greater reproducibility and sensitivity of AE testing. Introduction Acoustic emission is especially useful in testing of pressure vessels. Indeed, AE enables global and rapid testing of large structures, significantly reducing maintenance time and shutdown of facilities. Methods have changed over the last decades, moving from very traditional and diversified methods to more standardized ones. However, some tests are still currently performed according to procedures, which have more to do with the service provider's "reputation" rather than on a proven technique. The authorities responsible for the safety of facilities in France requested "uniformization" of AE testing methods for pressure vessels: this led to the creation of the Best Practices Guideline (Guide des Bonnes Pratiques GBP [1]), w